Compensation scheme for non-volatile memory

ABSTRACT

Methods for performing parallel voltage and current compensation during reading and/or writing of memory cells in a memory array are described. In some embodiments, the compensation may include adjusting a bit line voltage and/or bit line reference current applied to a memory cell based on a memory array zone, a bit line layer, and a memory cell direction associated with the memory cell. The compensation may include adjusting the bit line voltage and/or bit line reference current on a per memory cell basis depending on memory cell specific characteristics. In some embodiments, a read/write circuit for reading and/or writing a memory cell may select a bit line voltage from a plurality of bit line voltage options to be applied to the memory cell based on whether the memory cell has been characterized as a strong, weak, or typical memory cell.

CLAIM OF PRIORITY

The present application is a continuation of U.S. patent applicationSer. No. 13/773,078, entitled “Compensation Scheme for Non-VolatileMemory,” filed Feb. 21, 2013, which is herein incorporated by referencein its entirety.

BACKGROUND

Semiconductor memory is widely used in various electronic devices suchas cellular telephones, digital cameras, personal digital assistants,mobile computing devices, and non-mobile computing devices. Anon-volatile memory device (e.g., a flash memory device) allowsinformation to be stored and retained even when the non-volatile memorydevice is not connected to a source of power (e.g., a battery).Non-volatile memory devices typically include two-dimensional arrays ofnon-volatile memory cells. The memory cells within a two-dimensionalarray form a single layer of memory cells and may be selected viacontrol lines in the X and Y directions. Non-volatile memory devices mayalso include monolithic three-dimensional memory arrays in whichmultiple layers of memory cells are formed above a single substratewithout any intervening substrates. In recent years, non-volatile memorydevices have been scaled in order to reduce cost per bit. However, asprocess geometries shrink, many design and process challenges arepresented. These challenges include increased variability in memory cellI-V characteristics and increased word line and bit line resistance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A depicts one embodiment of a memory system.

FIG. 1B depicts one embodiment of memory core control circuits.

FIG. 1C depicts one embodiment of a memory core.

FIG. 1D depicts one embodiment of a memory bay.

FIG. 1E depicts one embodiment of a memory block.

FIG. 1F depicts another embodiment of a memory bay.

FIG. 2A depicts one embodiment of a schematic diagram corresponding withthe memory bay of FIG. 1F.

FIG. 2B depicts one embodiment of a schematic diagram corresponding witha memory bay arrangement wherein word lines and bit lines are sharedacross memory blocks, and both row decoders and column decoders aresplit.

FIG. 3A depicts one embodiment of a portion of a monolithicthree-dimensional memory array.

FIG. 3B depicts one embodiment of a portion of a monolithicthree-dimensional memory array that includes a second memory levelpositioned above a first memory level.

FIG. 3C depicts a subset of the memory array and routing layers of oneembodiment of a three-dimensional memory array.

FIG. 4 depicts one embodiment of a portion of a monolithicthree-dimensional memory array.

FIG. 5 depicts one embodiment of a read/write circuit along with aportion of a memory array.

FIG. 6A depicts one embodiment of a cross-point memory array.

FIG. 6B depicts an alternative embodiment of a cross-point memory array.

FIG. 7A depicts one embodiment of memory cell current distributions formemory cells programmed into an ON state and memory cells programmedinto an OFF state over an applied memory cell voltage.

FIG. 7B depicts one embodiment of memory cell current distributions formemory cells programmed into an ON and memory cells programmed into anOFF state for a given memory cell bias voltage applied to the memorycells.

FIG. 8A depicts one embodiment of circuitry for generating a pluralityof bit line voltage options.

FIG. 8B depicts one embodiment of a VSFG generator.

FIG. 8C depicts one embodiment of a read/write circuit.

FIG. 8D depicts one embodiment of a portion of a memory core.

FIG. 9A is a flowchart describing one embodiment of a process forprogramming a memory cell.

FIG. 9B is a flowchart describing one embodiment of a process forgenerating a plurality of bit line voltages associated with differentcompensation options.

FIG. 9C is a flowchart describing one embodiment of a process fordetermining whether a memory cell has characteristics of a strong memorycell, a weak memory cell, or a typical memory cell.

FIG. 9D is a flowchart describing one embodiment of a process forreading a memory cell.

DETAILED DESCRIPTION

Technology is described for performing parallel voltage and currentcompensation during reading and/or writing of memory cells in a memoryarray. In some embodiments, the compensation may include adjusting a bitline voltage and/or bit line reference current applied to a memory cellbased on a memory array zone, a bit line layer, and a memory celldirection associated with the memory cell. The compensation may includeadjusting the bit line voltage and/or bit line reference current on aper memory cell basis depending on memory cell specific characteristics.In some embodiments, a read/write circuit for reading and/or writing amemory cell may select a bit line voltage from a plurality of bit linevoltage options to be applied to the memory cell based on whether thememory cell has been characterized as a strong, weak, or typical memorycell. The read/write circuit may also select a bit line voltage of theplurality of bit line voltage options to be applied to the memory cellbased on whether the memory cell has been characterized as a stronglyconducting memory cell and whether the memory cell comprises a near-nearmemory cell.

One issue involving the programming of a memory cell is that, due tovariations in resistance along a programming path, the highest yieldingvoltage and/or current biasing conditions for the memory cell may varydepending whether the memory cell is a near-near memory cell (i.e., amemory cell with relatively small path resistance due to being locatednear the bit line biasing circuit and located near the word line biasingcircuit) or a far-far memory cell (i.e., a memory cell with relativelylarge path resistance due to being located far from the bit line biasingcircuit and located far from the word line biasing circuit). Arelatively small path resistance along the programming path through anear-near memory cell may lead to an increase in the number of resetfails. The number of reset fails may further increase if the near-nearmemory cell is a strongly conducting memory cell. To compensate for thisissue, in one example, a first memory cell comprising a near-near memorycell may use a reference current of 6 uA and a selected bit line voltageof 6V, while all other memory cells that do not comprise near-nearmemory cells may use a reference current of 4 uA and a selected bit linevoltage of 5.5V.

One issue involving the reading or verifying of a memory cell is thatthe highest yielding voltage and/or current biasing conditions for thememory cell may vary depending on a memory layer and/or an orientationof the memory cell. In one example, a first memory cell oriented in afirst direction may use a reference current of 3 uA and a selected bitline voltage of 2.1V, while a second memory cell oriented in a seconddirection may use a reference current of 1.2 uA and a selected bit linevoltage of 1.8V. Thus, there is a need to perform parallel voltage andcurrent compensation during reading and/or writing of memory cells on aper memory cell basis.

In some cases, a semiconductor memory array may include a cross-pointmemory array. A cross-point memory array comprises a memory array inwhich two-terminal memory cells are placed at the intersections of afirst set of control lines (e.g., word lines) arranged in a firstdirection and a second set of control lines (e.g., bit lines) arrangedin a second direction perpendicular to the first direction. Thetwo-terminal memory cells may include a phase change material. Eachmemory cell in a cross-point memory array may be placed in series with asteering element, such as a diode, in order to reduce leakage currentsassociated with unselected memory cells.

FIG. 1A depicts one embodiment of a memory system 100. Memory system 100includes a host 106 (e.g., a personal computer or mobile computingdevice) and a memory card 101. The memory card 101 includes a memorychip controller 105 and a memory chip 102. The memory chip controller105 may include one or more state machines, page registers, SRAM, orother control logic for controlling the operation of memory chip 102.The one or more state machines, page registers, SRAM, and/or othercontrol logic for controlling the operation of the memory chip may bereferred to as managing or control circuits for facilitating one or morememory array operations including erasing, programming, or readingoperations. The memory chip controller may receive data and commandsfrom host 106 and provides memory chip data to host 106.

In one embodiment, the memory chip controller 105 and memory chip 102may be arranged on a single integrated circuit. In other embodiments,memory chip controller 105 and memory chip 102 may be arranged ondifferent integrated circuits. The memory chip 102 includes memory corecontrol circuits 104 and a memory core 103. Memory core control circuits104 may include logic for controlling the selection of memory blocks (orarrays) within memory core 103, controlling the generation of voltagereferences for biasing a particular memory array into a read or writestate, or generating row and column addresses. The memory core 103 mayinclude one or more two-dimensional arrays of memory cells or one ormore three-dimensional arrays of memory cells. In one embodiment, thememory core control circuits 104 and memory core 103 are arranged on asingle integrated circuit. In other embodiments, the memory core controlcircuits 104 and memory core 103 are arranged on different integratedcircuits.

Referring to FIG. 1A, a memory card operation may be initiated when host106 sends instructions to memory chip controller 105 indicating that itwould like to read data from memory card 101 or write data to memorycard 101. In the event of a write (or programming) operation, host 106will send to memory chip controller 105 both a write command and thedata to be written. The data to be written may be buffered by memorychip controller 105 and error correcting code (ECC) data may begenerated corresponding with the data to be written. The ECC data, whichallows data errors that occur during transmission or storage to bedetected and/or corrected, may be written to memory core 103 or storedin non-volatile memory within memory chip controller 105. In oneembodiment, the ECC data is generated and data errors are corrected bycircuitry within memory chip controller 105.

As depicted in FIG. 1A, the operation of memory chip 102 may becontrolled by memory chip controller 105. In one example, before issuinga write operation to memory chip 102, memory chip controller 105 maycheck a status register to make sure that memory chip 102 is able toaccept the data to be written. In another example, before issuing a readoperation to memory chip 102, memory chip controller 105 may pre-readoverhead information associated with the data to be read. The overheadinformation may include ECC data associated with the data to be read ora redirection pointer to a new memory location within memory chip 102 inwhich to read the data requested. Once a read or write operation isinitiated by memory chip controller 105, memory core control circuits104 may generate the appropriate bias voltages for word lines and bitlines within memory core 103, as well as generate the appropriate memoryblock, row, and column addresses.

FIG. 1B depicts one embodiment of memory core control circuits 104. Asdepicted, the memory core control circuits 104 include address decoders170, voltage generators for selected control lines 172, and voltagegenerators for unselected control lines 174. Control lines may includeword lines, bit lines, or a combination of word lines and bit lines.Selected control lines may include selected word lines or selected bitlines that are used to place memory cells into a selected state.Unselected control lines may include unselected word lines or unselectedbit lines that are used to place memory cells into an unselected state.The voltage generators (or voltage regulators) for selected controllines 172 may comprise one or more voltage generators for generatingselected control line voltages. The voltage generators for unselectedcontrol lines 174 may comprise one or more voltage generators forgenerating unselected control line voltages. Address decoders 170 maygenerate memory block addresses, as well as row addresses and columnaddresses for a particular memory block.

FIGS. 1C-1F depict one embodiment of a memory core organization thatincludes a memory core having multiple memory bays, and each memory bayhaving multiple memory blocks. Although a memory core organization isdisclosed where memory bays comprise memory blocks, and memory blockscomprise a group of memory cells, other organizations or groupings canalso be used with the technology described herein.

FIG. 1C depicts one embodiment of memory core 103 in FIG. 1A. Asdepicted, memory core 103 includes memory bay 330 and memory bay 331. Insome embodiments, the number of memory bays per memory core can bedifferent for different implementations. For example, a memory core mayinclude only a single memory bay or a plurality of memory bays (e.g., 16memory bays).

FIG. 1D depicts one embodiment of memory bay 330 in FIG. 1C. Asdepicted, memory bay 330 includes memory blocks 310-312 and read/writecircuits 306. In some embodiments, the number of memory blocks permemory bay may be different for different implementations. For example,a memory bay may include one or more memory blocks (e.g., 32 memoryblocks per memory bay). Read/write circuits 306 include circuitry forreading and writing memory cells within memory blocks 310-312. Asdepicted, the read/write circuits 306 may be shared across multiplememory blocks within a memory bay. This allows chip area to be reducedsince a single group of read/write circuits 306 may be used to supportmultiple memory blocks. However, in some embodiments, only a singlememory block should be electrically coupled to read/write circuits 306at a particular time to avoid signal conflicts.

FIG. 1E depicts one embodiment of memory block 310 in FIG. 1D. Asdepicted, memory block 310 includes a memory array 301, row decoder 304,and column decoder 302. Memory array 301 may comprise a contiguous groupof memory cells having contiguous word lines and bit lines. Memory array301 may comprise one or more layers of memory cells (i.e., memory array310 may comprise a two-dimensional memory array or a three-dimensionalmemory array). The row decoder 304 decodes a row address and selects aparticular word line in memory array 301 when appropriate (e.g., whenreading or writing memory cells in memory array 301). The column decoder302 decodes a column address and selects a particular group of bit linesin memory array 301 to be electrically coupled to read/write circuits,such as read/write circuits 306 in FIG. 1D. In one embodiment, thenumber of word lines is 4K per memory layer, the number of bit lines is1K per memory layer, and the number of memory layers is 4, providing amemory array 301 containing 16M memory cells.

FIG. 1F depicts one embodiment of a memory bay 332. Memory bay 332 isone example of an alternative implementation for memory bay 330 in FIG.1D. In some embodiments, row decoders, column decoders, and read/writecircuits may be split or shared between memory arrays. As depicted, rowdecoder 349 is shared between memory arrays 352 and 354 because rowdecoder 349 controls word lines in both memory arrays 352 and 354 (i.e.,the word lines driven by row decoder 349 are shared). Row decoders 348and 349 may be split such that even word lines in memory array 352 aredriven by row decoder 348 and odd word lines in memory array 352 aredriven by row decoder 349. Column decoders 344 and 346 may be split suchthat even bit lines in memory array 352 are controlled by column decoder346 and odd bit lines in memory array 352 are driven by column decoder344. The selected bit lines controlled by column decoder 344 may beelectrically coupled to read/write circuits 340. The selected bit linescontrolled by column decoder 346 may be electrically coupled toread/write circuits 342. Splitting the read/write circuits intoread/write circuits 340 and 342 when the column decoders are splitallows for a more efficient layout of the memory bay.

FIG. 2A depicts one embodiment of a schematic diagram (including wordlines and bit lines) corresponding with memory bay 332 in FIG. 1F. Asdepicted, word lines WL1, WL3, and WL5 are shared between memory arrays352 and 354 and controlled by row decoder 349 of FIG. 1F. Word linesWL0, WL2, WL4, and WL6 are driven from the left side of memory array 352and controlled by row decoder 348 of FIG. 1F. Word lines WL14, WL16,WL18, and WL20 are driven from the right side of memory array 354 andcontrolled by row decoder 350 of FIG. 1F. Bit lines BL0, BL2, BL4, andBL6 are driven from the bottom of memory array 352 and controlled bycolumn decoder 346 of FIG. 1F. Bit lines BL1, BL3, and BL5 are drivenfrom the top of memory array 352 and controlled by column decoder 344 ofFIG. 1F.

In one embodiment, the memory arrays 352 and 354 may comprise memorylayers that are oriented in a horizontal plane that is horizontal to thesupporting substrate. In another embodiment, the memory arrays 352 and354 may comprise memory layers that are oriented in a vertical planethat is vertical to the supporting substrate (i.e., the vertical planeis perpendicular to the supporting substrate).

FIG. 2B depicts one embodiment of a schematic diagram (including wordlines and bit lines) corresponding with a memory bay arrangement whereinword lines and bit lines are shared across memory blocks, and both rowdecoders and column decoders are split. Sharing word lines and/or bitlines helps to reduce layout area since a single row decoder and/orcolumn decoder can be used to support two memory arrays. As depicted,word lines WL1, WL3, and WL5 are shared between memory arrays 406 and408. Bit lines BL1, BL3, and BL5 are shared between memory arrays 406and 402. Row decoders are split such that word lines WL0, WL2, WL4, andWL6 are driven from the left side of memory array 406 and word linesWL1, WL3, and WL5 are driven from the right side of memory array 406.Column decoders are split such that bit lines BL0, BL2, BL4, and BL6 aredriven from the bottom of memory array 406 and bit lines BL1, BL3, andBL5 are driven from the top of memory array 406. Splitting row and/orcolumn decoders also helps to relieve layout constraints (e.g., thecolumn decoder pitch can be relieved by 2× since the split columndecoders need only drive every other bit line instead of every bitline).

FIG. 3A depicts one embodiment of a portion of a monolithicthree-dimensional memory array 201 that includes a second memory level220 positioned above a first memory level 218. Memory array 201 is oneexample of an implementation for memory array 301 in FIG. 1E. The bitlines 206 and 210 are arranged in a first direction and the word lines208 are arranged in a second direction perpendicular to the firstdirection. As depicted, the upper conductors of first memory level 218may be used as the lower conductors of the second memory level 220 thatis positioned above the first memory level. In a memory array withadditional layers of memory cells, there would be correspondingadditional layers of bit lines and word lines.

As depicted in FIG. 3A, memory array 201 includes a plurality of memorycells 200. The memory cells 200 may include re-writeable memory cells.The memory cells 200 may include non-volatile memory cells or volatilememory cells. With respect to first memory level 218, a first portion ofmemory cells 200 are between and connect to bit lines 206 and word lines208. With respect to second memory level 220, a second portion of memorycells 200 are between and connect to bit lines 210 and word lines 208.In one embodiment, each memory cell includes a steering element (e.g., adiode) and a memory element (i.e., a state change element). In oneexample, the diodes of the first memory level 218 may be upward pointingdiodes as indicated by arrow A₁ (e.g., with p regions at the bottom ofthe diodes), while the diodes of the second memory level 220 may bedownward pointing diodes as indicated by arrow A₂ (e.g., with n regionsat the bottom of the diodes), or vice versa. In another embodiment, eachmemory cell includes only a state change element. The absence of a diodefrom a memory cell may reduce the process complexity and costsassociated with manufacturing a memory array.

In one embodiment, the memory cells 200 of FIG. 3A include re-writablenon-volatile memory cells. In one example, U.S. Patent ApplicationPublication No. 2006/0250836, which is herein incorporated by referencein its entirety, describes a rewriteable non-volatile memory cell thatincludes a diode coupled in series with a reversibleresistance-switching element. A reversible resistance-switching elementincludes reversible resistivity-switching material having a resistivitythat may be reversibly switched between two or more states. In oneembodiment, the reversible resistance-switching material may include ametal oxide. The metal oxide may include nickel oxide or hafnium oxide.In another embodiment, the reversible resistance-switching material mayinclude a phase change material. The phase change material may include achalcogenide material. In some cases, the re-writeable non-volatilememory cells may comprise resistive RAM (ReRAM) devices.

In another embodiment, the memory cells 200 of FIG. 3A may includeconductive bridge memory elements. A conductive bridge memory elementmay also be referred to as a programmable metallization cell. Aconductive bridge memory element may be used as a state change elementbased on the physical relocation of ions within a solid electrolyte. Insome cases, a conductive bridge memory element may include two solidmetal electrodes, one relatively inert (e.g., tungsten) and the otherelectrochemically active (e.g., silver or copper), with a thin film ofthe solid electrolyte between the two electrodes. As temperatureincreases, the mobility of the ions also increases causing theprogramming threshold for the conductive bridge memory cell to decrease.Thus, the conductive bridge memory element may have a wide range ofprogramming thresholds over temperature.

Referring to FIG. 3A, in one embodiment of a read operation, the datastored in one of the plurality of memory cells 200 may be read bybiasing one of the word lines (i.e., the selected word line) to theselected word line voltage in read mode (e.g., 0.4V). A read circuit isthen used to bias a selected bit line connected to the selected memorycell to the selected bit line voltage in read mode (e.g., 0V). In somecases, in order to avoid sensing leakage current from the manyunselected word lines to the selected bit line, the unselected wordlines may be biased to the same voltage as the selected bit lines (e.g.,0V). To avoid leakage current from the selected word line to theunselected bit lines, the unselected bit lines may be biased to the samevoltage as the selected word line (e.g., 0.4V); however, biasing theunselected word lines to the same voltage as the selected bit lines andbiasing the unselected bit lines to the same voltage as the selectedword line will place a large voltage stress across the unselected memorycells driven by both the unselected word lines and the unselected bitlines.

In an alternative read biasing scheme, both the unselected word linesand the unselected bit lines may be biased to an intermediate voltagethat is between the selected word line voltage and the selected bit linevoltage. Applying the same voltage to both the unselected word lines andthe unselected bit lines may reduce the voltage stress across theunselected memory cells driven by both the unselected word lines and theunselected bit lines; however, the reduced voltage stress comes at theexpense of increased leakage currents associated with the selected wordline and the selected bit line. Before the selected word line voltagehas been applied to the selected word line, the selected bit linevoltage may be applied to the selected bit line, and a read circuit maythen sense an auto zero amount of current through the selected memorybit line which is subtracted from the bit line current in a secondcurrent sensing when the selected word line voltage is applied to theselected word line. Leakage current may be subtracted out by using theauto zero current sensing.

Referring to FIG. 3A, in one embodiment of a write operation, thereversible resistance-switching material may be in an initialhigh-resistivity state that is switchable to a low-resistivity stateupon application of a first voltage and/or current. Application of asecond voltage and/or current may return the reversibleresistance-switching material back to the high-resistivity state.Alternatively, the reversible resistance-switching material may be in aninitial low-resistance state that is reversibly switchable to ahigh-resistance state upon application of the appropriate voltage(s)and/or current(s). When used in a memory cell, one resistance state mayrepresent a binary data “0” while another resistance state may representa binary data “1.” In some cases, a memory cell may be considered tocomprise more than two data/resistance states (i.e., a multi-levelmemory cell). In some cases, a write operation is similar to a readoperation except with a larger voltage range placed across the selectedmemory cells.

The process of switching the resistance of a reversibleresistance-switching element from a high-resistivity state to alow-resistivity state may be referred to as SETTING the reversibleresistance-switching element. The process of switching the resistancefrom the low-resistivity state to the high-resistivity state may bereferred to as RESETTING the reversible resistance-switching element.The high-resistivity state may be associated with binary data “0” andthe low-resistivity state may be associated with binary data “1.” Inother embodiments, SETTING and RESETTING operations and/or the dataencoding can be reversed. In some embodiments, the first time aresistance-switching element is SET requires a higher than normalvoltage and is referred to as a FORMING operation.

Referring to FIG. 3A, in one embodiment of a write operation, data maybe written to one of the plurality of memory cells 200 by biasing one ofthe word lines (i.e., the selected word line) to the selected word linevoltage in write mode (e.g., 5V). A write circuit may be used to biasthe bit line connected to the selected memory cell to the selected bitline voltage in write mode (e.g., 0V). In some cases, in order toprevent program disturb of unselected memory cells sharing the selectedword line, the unselected bit lines may be biased such that a firstvoltage difference between the selected word line voltage and theunselected bit line voltage is less than a first disturb threshold. Toprevent program disturb of unselected memory cells sharing the selectedbit line, the unselected word lines may be biased such that a secondvoltage difference between the unselected word line voltage and theselected bit line voltage is less than a second disturb threshold. Thefirst disturb threshold and the second disturb threshold may bedifferent depending on the amount of time in which the unselected memorycells susceptible to disturb are stressed.

In one write biasing scheme, both the unselected word lines and theunselected bit lines may be biased to an intermediate voltage that isbetween the selected word line voltage and the selected bit linevoltage. The intermediate voltage may be generated such that a firstvoltage difference across unselected memory cells sharing a selectedword line is greater than a second voltage difference across otherunselected memory cells sharing a selected bit line. One reason forplacing the larger voltage difference across the unselected memory cellssharing a selected word line is that the memory cells sharing theselected word line may be verified immediately after a write operationin order to detect a write disturb.

FIG. 3B depicts one embodiment of a portion of a monolithicthree-dimensional memory array that includes a second memory levelpositioned above a first memory level. The second memory level includesmemory element 224 and steering element 226. The first memory levelincludes memory element 228 and steering element 230. The bit lines 206and 210 are arranged in a first direction and the word line 208 isarranged in a second direction perpendicular to the first direction. Asdepicted, the memory element 224 may include a memory layer stackcomprising a layer of p-type polycrystalline silicon (or polysilicon)formed above a layer of hafnium oxide. The steering element 226 maycomprise a diode (e.g., a L1 diode) pointing in a first direction. Thememory element 228 may include a memory layer stack comprising a layerof hafnium oxide formed above a layer of p-type polysilicon. Thesteering element 230 may comprise a diode (e.g., a L0 diode) pointing ina second direction different from the first direction. The bit lines 206and 210 and the word line 208 may comprise a tungsten layer. In somecases, titanium nitride layers may be used as barrier layers (e.g.,formed between a memory element and a steering element) or adhesionlayers (e.g., formed above a word line layer or a bit line layer).

FIG. 3C depicts a subset of the memory array and routing layers of oneembodiment of a three-dimensional memory array, such as memory array 301in FIG. 1E. As depicted, the Memory Array layers are positioned abovethe Substrate. The Memory Array layers include bit line layers BL0, BL1and BL2, and word line layers WL0 and WL1. In other embodiments,additional bit line and word line layers can also be implemented.Supporting circuitry (e.g., row decoders, column decoders, andread/write circuits) may be arranged on the surface of the Substratewith the Memory Array layers fabricated above the supporting circuitry.An integrated circuit implementing a three-dimensional memory array mayalso include multiple metal layers for routing signals between differentcomponents of the supporting circuitry, and between the supportingcircuitry and the bit lines and word lines of the memory array. Theserouting layers can be arranged above the supporting circuitry that isimplemented on the surface of the Substrate and below the Memory Arraylayers.

As depicted in FIG. 3C, two metal layers R1 and R2 are used for routinglayers; however, other embodiments can include more or less than twometal layers. In one example, these metal layers R1 and R2 are formed oftungsten (about 1 ohm/square). Positioned above the Memory Array layersmay be one or more top metal layers used for routing signals betweendifferent components of the integrated circuit such as the Top Metallayer. In one example, the Top Metal layer is formed of copper oraluminum (about 0.05 ohms/square), which may provide a smallerresistance per unit area than layers R1 and R2. Metals layers R1 and maynot be implemented using the same materials as those used for the TopMetal layers because the metal used for R1 and R2 must be able towithstand the processing steps for fabricating the Memory Array layerson top of R1 and R2.

FIG. 4 depicts one embodiment of a portion of a monolithicthree-dimensional memory array 402 that includes a first memory level412 positioned below a second memory level 410. Memory array 402 is oneexample of an implementation for memory array 301 in FIG. 1E. The localbit lines LBL₁₁-LBL₃₃ are arranged in a first direction (i.e., avertical direction) and the word lines WL₁₀-WL₂₃ are arranged in asecond direction perpendicular to the first direction. This arrangementof vertical bit lines in a monolithic three-dimensional memory array isone embodiment of a vertical bit line memory array. As depicted,disposed between the intersection of each local bit line and each wordline is a particular memory cell (e.g., memory cell M₁₁₁ is disposedbetween local bit line LBL₁₁ and word line WL₁₀). The global bit linesGBL₁-GBL₃ are arranged in a third direction that is perpendicular toboth the first direction and the second direction. A set of bit lineselect devices (e.g., Q₁₁-Q₃₁) may be used to select a set of local bitlines (e.g., LBL₁₁-LBL₃₁). As depicted, bit line select devices Q₁₁-Q₃₁are used to select the local bit lines LBL₁₁-LBL₃₁ and to connect thelocal bit lines LBL₁₁-LBL₃₁ to the global bit lines GBL₁-GBL₃ using rowselect line SG₁. Similarly, bit line select devices Q₁₂-Q₃₂ are used toselectively connect the local bit lines LBL₁₂-LBL₃₂ to the global bitlines GBL₁-GBL₃ using row select line 502 and bit line select devicesQ₁₃-Q₃₃ are used to selectively connect the local bit lines LBL₁₃-LBL₃₃to the global bit lines GBL₁-GBL₃ using row select line SG3.

Referring to FIG. 4, as only a single bit line select device is used perlocal bit line, only the voltage of a particular global bit line may beapplied to a corresponding local bit line. Therefore, when a first setof local bit lines (e.g., LBL₁₁-LBL₃₁) is biased to the global bit linesGBL₁-GBL₃, the other local bit lines (e.g., LBL₁₂-LBL₃₂ and LBL₁₃-LBL₃₃)must either also be driven to the same global bit lines GBL₁-GBL₃ or befloated. In one embodiment, during a memory operation, all local bitlines within the memory array are first biased to an unselected bit linevoltage by connecting each of the global bit lines to one or more localbit lines. After the local bit lines are biased to the unselected bitline voltage, then only a first set of local bit lines LBL₁₁-LBL₃₁ arebiased to one or more selected bit line voltages via the global bitlines GBL₁-GBL₃, while the other local bit lines (e.g., LBL₁₂-LBL₃₂ andLBL₁₃-LBL₃₃) are floated. The one or more selected bit line voltages maycorrespond with, for example, one or more read voltages during a readoperation or one or more programming voltages during a programmingoperation.

In one embodiment, a vertical bit line memory array, such as memoryarray 402, includes a greater number of memory cells along the wordlines as compared with the number of memory cells along the bit lines.For example, the number memory cells along each bit line may be 16,while the number of memory cells along each word line may be 2048. Moreinformation regarding the structure and operation of vertical bit linememory arrays can be found in U.S. Provisional Application 61/423,007,“Non-Volatile Memory Having 3D Array of Read/Write Elements WithVertical Bit Lines and Laterally Aligned Active Elements and MethodsThereof” and U.S. patent application Ser. No. 13/323,703, “ThreeDimensional Non-Volatile Storage with Three Device Driver for RowSelect,” both of which are herein incorporated by reference in theirentirety.

FIG. 5 depicts one embodiment of a read/write circuit 502 along with aportion of a memory array 501. Read/write circuit 502 is one example ofan implementation of read/write circuit 306 in FIG. 1D. The portion of amemory array 501 includes two of the many bit lines (one selected bitline labeled “Selected BL” and one unselected bit line labeled“Unselected BL”) and two of the many word lines (one selected word linelabeled “Selected WL” and one unselected word line labeled “UnselectedWL”). The portion of a memory array also includes a selected memory cell550 and unselected memory cells 552-556. In one embodiment, the portionof a memory array 501 may comprise a memory array with bit linesarranged in a direction horizontal to the substrate, such as memoryarray 201 in FIG. 3A. In another embodiment, the portion of a memoryarray 501 may comprise a memory array with bit lines arranged in avertical direction that is perpendicular to the substrate, such asmemory array 402 in FIG. 4. As depicted, the selected bit line is biasedto 1V, the unselected word line is biased to 0.6V, the selected wordline is biased to 0V, and the unselected bit line is biased to 0.5V.

In some embodiments, the selected bit line may be biased to 2.0V, theunselected word line may be biased to 2.0V, the selected word line maybe biased to 0V, and the unselected bit line may be biased to 0V. Inother embodiments, the memory array biasing scheme of FIG. 5 may bereversed such that the selected bit line is biased to 0V, the unselectedword line is biased to 0.4V, the selected word line is biased to 1V, andthe unselected bit line is biased to 0.5V.

As depicted in FIG. 5, the SELB node of read/write circuit 502 iselectrically coupled to the selected bit line via column decoder 504. Inone embodiment, column decoder 504 may correspond with column decoder302 depicted in FIG. 1E. Transistor 562 couples node SELB to the Vsensenode. The transistor 562 may comprise a low VT nMOS device. Clampcontrol circuit 564 controls the gate of transistor 562. The Vsense nodeis connected to reference current Iref and one input of sense amplifier566. The other input of sense amplifier 566 receives Vref-read, which isthe voltage level used for comparing the Vsense node voltage in readmode. The output of sense amplifier 566 is connected to the data outterminal and to data latch 568. Write circuit 560 is connected to nodeSELB, the data in terminal, and data latch 568.

During a read operation, read/write circuit 502 biases the selected bitline to the selected bit line voltage in read mode. Prior to sensingdata, read/write circuit 502 will precharge the Vsense node to 2V. Whensensing data, read/write circuit 502 attempts to regulate the SELB nodeto 1V via clamp control circuit 564 and transistor 562 in asource-follower configuration. If the current through the selectedmemory cell 550 is greater than the read current limit, Iref, then, overtime, the Vsense node will fall below Vref-read (e.g., set to 1.5V) andthe sense amplifier 566 will read out a data “0.” Outputting a data “0”represents that the selected memory cell 550 is in a low resistancestate (e.g., a SET state). If the current through the selected memorycell 550 is less than Iref, then the Vsense node will stay aboveVref-read and the sense amplifier 566 will read out a data “1.”Outputting a data “1” represents that the selected memory cell 550 is ina high resistance state (e.g., a RESET state). Data latch 568 will latchthe output of sense amplifier 566 after a time period of sensing thecurrent through the selected memory cell (e.g., 400 ns).

In one embodiment, during a write operation, if the data in terminalrequests a data “0” to be written to a selected memory cell, thenread/write circuit 502 biases SELB to the selected bit line voltage inwrite mode (e.g., 1.2V for a SET operation) via write circuit 560. Theduration of programming the memory cell can be a fixed time period(e.g., using a fixed-width programming pulse) or variable (e.g., using awrite circuit 560 that senses whether a memory cell has been programmedwhile programming). More information regarding write circuits that cansense while programming data can be found in U.S. Pat. No. 6,574,145,“Memory Device and Method for Sensing While Programming a Non-VolatileMemory Cell,” incorporated herein by reference in its entirety. If thedata in terminal requests a data “1” to be written, then write circuit560 may bias SELB to the unselected bit line voltage in write mode(e.g., 0V for a SET operation). The write circuit 560 may also bias SELBto a program inhibit voltage in write mode that is different from theunselected bit line voltage.

FIG. 6A depicts one embodiment of a cross-point memory array 610. Thecross-point memory array 610 may correspond with memory array 201 inFIG. 3A or memory array 402 in FIG. 4. As depicted, cross-point memoryarray 610 includes word lines 602-608 and bit lines 612-618. Word line604 comprises a selected word line and bit line 614 comprises a selectedbit line. At the intersection of selected word line 604 and selected bitline 614 is a selected memory cell (an S cell). The voltage across the Scell is the difference between the selected word line voltage and theselected bit line voltage. Memory cells at the intersections of theselected word line 604 and the unselected bit lines 612, 616, and 618comprise unselected memory cells (H cells). H cells are unselectedmemory cells that share a selected word line that is biased to theselected word line voltage. The voltage across the H cells is thedifference between the selected word line voltage and the unselected bitline voltage. Memory cells at the intersections of the selected bit line614 and the unselected word lines 602, 606, and 608 comprise unselectedmemory cells (F cells). F cells are unselected memory cells that share aselected bit line that is biased to a selected bit line voltage. Thevoltage across the F cells is the difference between the unselected wordline voltage and the selected bit line voltage. Memory cells at theintersections of the unselected word lines 602, 606, and 608 and theunselected bit lines 612, 616, and 618 comprise unselected memory cells(U cells). The voltage across the U cells is the difference between theunselected word line voltage and the unselected bit line voltage.

The number of F cells is related to the length of the bit lines (or thenumber of memory cells connected to a bit line) while the number of Hcells is related to the length of the word lines (or the number ofmemory cells connected to a word line). The number of U cells is relatedto the product of the word line length and the bit line length. In oneembodiment, each memory cell sharing a particular word line, such asword line 602, may be associated with a particular page stored withinthe cross-point memory array 610.

FIG. 6B depicts an alternative embodiment of a cross-point memory array620. The cross-point memory array 620 may correspond with memory array201 in FIG. 3A or memory array 402 in FIG. 4. As depicted, cross-pointmemory array 620 includes word lines 622-628 and bit lines 632-638. Wordline 624 comprises a selected word line and bit lines 634 and 638comprise selected bit lines. Although both bit lines 634 and 638 areselected, the voltages applied to bit line 634 and bit line 638 may bedifferent. For example, in the case that bit line 634 is associated witha first memory cell to be programmed (i.e., an S cell), then bit line634 may be biased to a selected bit line voltage in order to program thefirst memory cell. In the case that bit line 638 is associated with asecond memory cell that is not to be programmed (i.e., an I cell), thenbit line 638 may be biased to a program inhibit voltage (i.e., to a bitline voltage that will prevent the second memory cell from beingprogrammed).

At the intersection of selected word line 624 and selected bit line 638is a program inhibited memory cell (an I cell). The voltage across the Icell is the difference between the selected word line voltage and theprogram inhibit voltage. Memory cells at the intersections of theselected bit line 638 and the unselected word lines 622, 626, and 628comprise unselected memory cells (X cells). X cells are unselectedmemory cells that share a selected bit line that is biased to a programinhibit voltage. The voltage across the X cells is the differencebetween the unselected word line voltage and the program inhibitvoltage. In one embodiment, the program inhibit voltage applied to theselected bit line 638 may be similar to the unselected bit line voltage.In another embodiment, the program inhibit voltage may be a voltage thatis greater than or less than the unselected bit line voltage. Forexample, the program inhibit voltage may be set to a voltage that isbetween the selected word line voltage and the unselected bit linevoltage. In some cases, the program inhibit voltage applied may be afunction of temperature. In one example, the program inhibit voltage maytrack the unselected bit line voltage over temperature.

In one embodiment, two or more pages may be associated with a particularword line. In one example, word line 622 may be associated with a firstpage and a second page. The first page may correspond with bit lines 632and 636 and the second page may correspond with bit lines 634 and 638.In this case, the first page and the second page may correspond withinterdigitated memory cells that share the same word line. When a memoryarray operation is being performed on the first page (e.g., aprogramming operation) and the selected word line 624 is biased to theselected word line voltage, one or more other pages also associated withthe selected word line 624 may comprise H cells because the memory cellsassociated with the one or more other pages will share the same selectedword line as the first page.

In some embodiments, not all unselected bit lines may be driven to anunselected bit line voltage. Instead, a number of unselected bit linesmay be floated and indirectly biased via the unselected word lines. Inthis case, the memory cells of memory array 620 may comprise resistivememory elements without isolating diodes. In another embodiment, afloating control line (e.g., bit line 636) comprises a portion of thememory array that may be undriven during an operation on memory cell Susing a first selected control line (e.g., bit line 634). Selectiondevices connected to control line 636 may be turned off during thememory operation causing control line 636 to be floating. Since aportion of the memory cells connected to the control lines 634 and 636are also connected to shared unselected second control lines 622, 626,and 628, the floating control lines will float to a voltagesubstantially the same as the voltage of the unselected second controllines. In one embodiment, the control lines 634 and 636 may comprisevertical bit lines in a three dimensional memory array comprising combshaped word lines. More information regarding vertical bit line threedimensional memory arrays can be found in U.S. Provisional Application61/526,764, “Optimized Architecture for Three Dimensional Non-VolatileStorage Device with Vertical Bit Lines” and U.S. patent application Ser.No. 13/323,573, “Three Dimensional Non-Volatile Storage with Multi BlockRow Selection,” both of which are herein incorporated by reference intheir entirety.

FIG. 7A depicts one embodiment of memory cell current distributions formemory cells programmed into an ON state (bounded by the lines 702-703)and memory cells programmed into an OFF state (bounded by lines 712-713)over an applied memory cell voltage (VCELL). As depicted, the memorycell current distribution for memory cells programmed into the ON state(e.g., one of the possible conducting states for a memory cell) is shownby a range bounded by lines 702-703, which comprise the boundaries ofthe expected variability for a given probability distribution (e.g.,+/−4 sigma variation). In one example, the line 702 may correspond withmemory cell I-V characteristics associated with a “strong” ON memorycell and the line 703 may correspond with memory cell I-Vcharacteristics associated with a “weak” ON memory cell. The memory cellcurrent distribution for memory cells programmed into the OFF state isshown by a range bounded by lines 712-713, which comprise the boundariesof the expected variability for a given probability distribution (e.g.,+/−4 sigma variation). In one example, the line 712 may correspond withmemory cell I-V characteristics associated with a “strong” OFF memorycell and the line 713 may correspond with memory cell I-Vcharacteristics associated with a “weak” OFF memory cell.

As depicted, for a given VCELL (e.g., 1.7V applied across a memorycell), the output current (ICELL) associated with a “weak” ON memorycell is greater than the ICELL associated with a “strong” OFF memorycell. However, if the voltage applied across a “weak” ON memory cell isless than the voltage applied across a “strong” OFF memory cell (e.g.,due to IR voltage drops along a bit line or a word line), then the ICELLassociated with the “weak” ON memory cell may not be greater than theICELL associated with the “strong” OFF memory cell. For example, if theVCELL applied to a “strong” OFF memory cell is 1.7V and the VCELLapplied to a “weak” ON memory cell is 1.5V, then the output currents ofboth cells may be roughly equal; thus, in this case, the states of thetwo memory cells may not be distinguishable.

FIG. 7B depicts one embodiment of memory cell current distributions formemory cells programmed into an ON state (corresponding with memory cellcurrent distribution 760) and memory cells programmed into an OFF state(corresponding with memory cell current distribution 762) for a givenmemory cell bias voltage applied to the memory cells (e.g., 1.7V appliedacross the memory cells). The ON state memory cells associated withoutput currents above an ICELL level of IC may be deemed “strong” ONmemory cells and the ON state memory cells associated with outputcurrents below an ICELL level of IB may be deemed “weak” ON memorycells. The ON state memory cells associated with output currents betweenthe ICELL levels IB and IC may be deemed “typical” ON memory cells.Similarly, the OFF state memory cells associated with output currentsabove an ICELL level of IA may be deemed “strong” OFF memory cells.

In one embodiment, a particular memory cell may be determined to by inan ON state corresponding with the memory cell current distribution 760using one or more sensing operations. To determine whether theparticular memory cell is at a low end of a current distribution 760associated with “weak” memory cells, an output current comparison withcurrent level IB may be performed. To determine whether the particularmemory cell is at a high end of the current distribution 760 associatedwith “strong” memory cells, an output current comparison with currentlevel IC may be performed. Similarly, to determine whether a particularmemory cell is at a high end of the current distribution 762 associatedwith “strong” memory cells, an output current comparison with currentlevel IA may be performed.

FIG. 8A depicts one embodiment of circuitry for generating a pluralityof bit line voltage options. Each voltage of the plurality of bit linevoltage options may be used for generating a different selected bit linevoltage. As depicted, the VBL options generator 801 includes a VBLsettings generator 802, VBL generators 804-807, and VSFG generators812-815. Each of the VBL generators 804-807 may comprise a non-invertingamplifier with a configurable resistor network (or ladder) that may beconfigured based on an input voltage setting. One example of a VSFGgenerator is depicted in FIG. 8B. The VBL settings generator 802 mayinclude mapping logic or a state machine for generating a plurality ofsettings associated with a plurality of bit line voltages. In oneembodiment, an uncompensated (or baseline) bit line voltage setting,BL_voltage_setting, may comprise a digital value used as theuncompensated voltage setting from which the plurality of settings maybe determined. In one example, BL_voltage_setting may comprise a binarynumber associated with a selected bit line voltage during a readoperation (e.g., 2V).

As depicted, the VBL settings generator 802 may take as inputs a memoryaddress (e.g., comprising row and column addresses associated with amemory array), and various offset voltage settings includingBL_zone_offset (an offset based on zones of a memory array forcompensating for bit line resistance), BL_layer_offset (an offset basedon the bit line layer), Cell_direction_offset (an offset based on thedirection of a memory element or steering element), andEdge_array_offset (an offset based on whether a memory array is near anedge of a memory die). The memory address may be generated using anaddress decoder, such as address decoders 170 in FIG. 1B, and mayinclude row address information and column address information fortargeted memory cells in a memory array. The memory address may be usedto determine the targeted memory array zone, the bit line layer, thememory cell direction, and whether the target memory array is an edgearray. In some embodiments, a look-up table or combinational logic maybe used to determine a compensated bit line voltage based on the memoryaddress and the various offset voltage settings. In one example, amemory address corresponding with a near zone in a memory array, a thirdbit line layer, and an up cell direction may cause a BL_voltage_settingset to 2.0V to be mapped to a compensated bit line voltage of 2.25V.

As depicted, the VBL settings generator 802 may take as inputsWL_near_far_offset (offset values associated with whether a memory cellis located at a near end of a word line or a far end of the word line)and Cell_weak_strong_offset (offset values associated with whether amemory cell is a “strong” memory cell or a “weak” memory cell).

In one embodiment, if a memory cell is located at a far end of a wordline, then the bit line voltage associated with the memory cell may beincreased to compensate for additional IR drops along the word line. Asboth “near” bits and “far” bits may be read during a read operation orprogrammed during a programming operation, two compensated bit linevoltages may be generated corresponding with a first compensated voltageto be applied to memory cells located at a near end of a word line and asecond compensated voltage to be applied to memory cells located at afar end of the word line.

In one embodiment, if a memory cell is deemed to be a “weak” memorycell, then the bit line voltage associated with the memory cell may beincreased to compensate for the reduced output current capability of thememory cell. As the determination of whether a memory cell is a “strong”memory cell or a “weak” memory cell must be made on a per memory cellbasis, two compensated bit line voltages may be generated correspondingwith a first compensated voltage to be applied to “strong” memory cellsand a second compensated voltage to be applied to “weak” memory cells.

In some embodiments, four compensated bit line voltages (e.g.,corresponding with VBL1-VBL4) may be generated corresponding with thefour memory cell combinations associated with near/far memory cells andstrong/weak memory cells. The VBL settings generator 802 may outputbinary values associated with the four compensated bit line voltages(e.g., VBD1_setting, VBL2_setting, VBL3_setting, and VBL4_setting) tofour voltage generators for generating the four compensated bit linevoltages. In some cases, the four compensated bit line voltagesVBL1-VBL4 may be distributed to memory cell write circuitry for biasingselected bit lines during a programming operation.

In some embodiments, rather than outputting (or distributing) the fourcompensated bit line voltages directly to read/write circuits, such asread/write circuits 306 in FIG. 1D, four source-follower gate (SFG)voltages may be generated for driving transistors in a source-followerconfiguration for biasing selected bit lines, such as transistor 562 inFIG. 5.

FIG. 8B depicts one embodiment of a VSFG generator, such as VSFGgenerator 812 in FIG. 8A. The VSFG generator includes amplifier 822,transistor 824, amplifier 826, and reference current 821. The referencecurrent 821 may be implemented using a current mirror and may be set toa value related to a minimum sensing current associated with a memorycell. As depicted, due to closed-loop feedback through amplifier 822,the source of transistor 824 may be regulated close to the input voltageVBL that is input to the amplifier 822. This will cause the gate oftransistor 824 to be biased to an appropriate SFG voltage such thattransistor 824 may source current associated with the reference current821 using a source voltage close to the input voltage VBL. Amplifier 826in a unity gain amplifier configuration may be used as a buffer to drivethe output voltage VBL_SFG to one or more transistors, such astransistor 562 and FIG. 5.

FIG. 8C depicts one embodiment of a read/write circuit 852 for readingand/or writing a particular memory cell selected via a column decoder.Read/write circuit 852 is one example of an implementation of aread/write circuit included in read/write circuits 306 in FIG. 1D. Insome embodiments, the capacitance associated with the SELB node (e.g., 3pF) may be larger than the capacitance associated with the Vsense node(e.g., 100 fF) due to wiring capacitance and diffusion capacitanceassociated with connecting the SELB node to numerous column decoders.The read/write circuit 852 may comprise a memory cell sensing circuitfor determining the state of a memory cell during a sensing operation.The read/write circuit 852 may also comprise a memory cell programmingcircuit for programming a memory cell using a bit line voltage selectedfrom plurality of bit line voltage options.

As depicted, the SELB node of read/write circuit 852 is connected to thesource of transistor 862 and write circuit 860. Transistor 862 couplesnode SELB to the Vsense node. Multiplexor (mux) 856 controls the gate oftransistor 862, which may comprise a low VT nMOS device. The Vsense nodeis connected to the drain of transistor 862, a reference current Iref(which may be configurable), and one input of sense amplifier 866. Theother input of sense amplifier 866 receives Vref, which is the voltagelevel used for comparing the Vsense node voltage during read and/orprogramming operations. The output of sense amplifier 866 is connectedto the data out terminal and to data latch 868. Write circuit 860 isconnected to node SELB, the data in terminal, and data latch 868.

As depicted, mux 856 takes as inputs four voltages VBD_SFG, VBL2_SFG,VBL3_SFG, and VBL4_SFG associated with four different selected bit linevoltages. The mux 856 selects one of the four input voltages to beconnected to the gate of transistor 862 based on two select inputsBL/WL_address (e.g., address information associated with whether aselected memory cell is a “near” bit or a “far” bit) andCell_weak_strong (e.g., a single bit value associated with whether aselected memory cell is a “strong” memory cell or a “weak” memory cell).

In one embodiment, four compensated bit line voltages are provided to aplurality of read/write circuits for per bit selection of theappropriate bit line voltage. In one example, during a programmingoperation, a read/write circuit may select the appropriate bit linevoltage to apply based on whether the targeted memory cell is in a nearzone or a far zone and/or whether the target memory cell is a “strong”memory cell or a “weak” memory cell. In another embodiment, four SFGvoltages are provided to a plurality of read/write circuits for per bitselection of the appropriate SFG voltage used for applying anappropriate bit line voltage to a selected bit line. In some cases, mux856 may be used for selecting an appropriate SFG voltage for drivingtransistor 862 during a read operation. In other cases, mux 856 may beused for selecting an appropriate SFG voltage for driving transistor 862during a programming operation. During a programming operation, thewrite circuit 860 may also directly apply a selected bit line voltagefrom a plurality of bit line voltage options via a mux not depicted.

FIG. 8D depicts one embodiment of a portion of a memory core, such asmemory core 103 in FIG. 1A. The memory core includes a top stripe, amemory array 890, and a bottom stripe. The top stripe includes a firstVBL options generator 801 for generating four voltages VBL1-VBL4, a pageregister 881 (or data buffer), read/write circuits 883, and columndecoder 885. The bottom stripe includes a second VBL options generator801 for generating four voltages VBL5-VBL8, a page register 882 (or databuffer), read/write circuits 884, and column decoder 886. In some cases,the column decoder 885 may be used for selecting even bit lines and thecolumn decoder 886 may be used for selecting odd bit lines.

As depicted, the memory array 890 may be broken into zones. In oneexample, each zone may be determined based on a range of row addressesor a range of word lines. The memory array 890 may be grouped into Nzones, where Zone 1 is closest to read/write circuits 883 and Zone N isclosest to read/write circuits 884. In this case, due to bit lineresistance, memory cells located in Zone N may experience a larger IRdrop along bit lines driven from read/write circuits 883 than memorycells located in Zone 1. Similarly, due to bit line resistance, memorycells located in Zone N may experience a smaller IR drop along bit linesdriven from read/write circuits 884 than memory cells located in Zone 1.In one embodiment, the top stripe bit line voltages corresponding withVBL1-VBL4 may be different from the bottom stripe bit line voltagescorresponding with VBL5 -VBL8 to compensate for bit line resistancebased on a target zone for targeted memory cells during a read and/orwrite operation. In one example, during a programming operation,VBL1-VBL4 may correspond with the voltages 6.0V, 6.15V, 6.3V, and 6.45V,respectively, while VBL5-VBL8 may correspond with the voltages 6.45V,6.6V, 6.75V, and 6.9V, respectively

FIG. 9A is a flowchart describing one embodiment of a process forprogramming a memory cell. In one embodiment, the process of FIG. 9A isperformed by a memory chip, such as memory chip 102 in FIG. 1.

In step 902, a plurality of bit line voltage options associated with awrite operation is generated. The plurality of bit line voltage optionsmay be generated using a bit line voltage options generator, such as VBLoptions generator 801 in FIG. 8A. In one embodiment, the plurality ofbit line voltage options may comprise four different bit line voltagescorresponding with the four memory cell combinations associated withnear/far memory cells and strong/weak memory cells. One embodiment of aprocess for generating a plurality of bit line voltages associated withdifferent compensation options is described later in reference to FIG.9B.

In step 904, a plurality of bit line current options associated with thewrite operation is generated. The plurality of bit line current optionsmay be generated using control circuitry similar to the VBL settingsgenerator 802 of FIG. 8A, but instead of binary values for differentvoltage settings being generated, the binary values may be generated fordifferent current settings. A plurality of reference currentscorresponding with the different current settings may be generated usinga plurality of configurable current mirrors. Each of the plurality ofconfigurable current mirrors may multiply a stable reference current(e.g., provided by a bandgap-based current reference) by some valuedepending on an input current setting. In one embodiment, the pluralityof bit line current options may comprise two different bit line currentscorresponding with a strong memory cell and a weak memory cell.

In step 906, it is determined whether a memory cell is associated withan output current that is above an upper threshold given an appliedvoltage across the memory cell. The memory cell may comprise one of aplurality of memory cells to be programmed during the write operation.In one embodiment, the determination of whether the memory cell isassociated with an output current that is above an upper thresholdincludes determining a state of the memory cell, determining an uppercurrent level associated with a “strong” memory cell in the state, andthen comparing the output current with the upper current levelassociated with a “strong” memory cell in the state. If the outputcurrent is greater than the upper current level, then the memory cellmay be deemed to be a “strong” memory cell.

In another embodiment, the determination of whether the memory cell isassociated with an output current that is above an upper thresholdincludes determining whether the memory cell is an ON state, determiningan upper current level associated with the ON state, and then comparingthe output current of the memory cell with the upper current level. Thedetermination of whether a memory cell to be written comprises a strong,weak, or typical memory cell may be performed for each of the memorycells to be written during the write operation. The memory cell specificdeterminations may then be stored as a memory cell strength vector in amemory buffer, such as a page register, prior to performing the writeoperation.

One embodiment of a process for determining whether a memory cell hascharacteristics of a strong memory cell, weak memory cell, or a typicalmemory cell is described later in reference to FIG. 9C.

In step 908, it is determined whether the memory cell is associated witha near end of a word line. In one embodiment, the determination ofwhether the memory cell is located at a near end of the word line may bedetermined based on a word line address associated with the memory cell.In some cases, a memory cell may be deemed to be at a near end of a wordline if it is located within a certain distance of a word line driverdriving the word line. In other embodiments, it is determined whetherthe memory cell is associated with a near-near memory cell (i.e., amemory cell with relatively small path resistance due to being locatednear the bit line biasing circuit and located near the word line biasingcircuit) based on a word line address and a bit line address associatedwith the memory cell.

In step 910, a first bit line voltage of the plurality of bit linevoltage options is selected. In one embodiment, the first bit linevoltage may be selected based on the determination that the memory cellcomprises a “strong” memory cell. In another environment, the first bitline voltage may be selected based on the determination that the memorycell comprises a “strong” memory cell and is located within apredetermined distance from a word line driver driving a word lineconnected to the memory cell (e.g., the memory cell comprises a near bitbased on the word line address).

In some cases, a page register may store a one bit or two bit vectorassociated with characteristics of the memory cell, such as whether thememory cell has been determined to be a strong, weak, or typical memorycell. The selection of the first bit line voltage may be performed usinga read/write circuit, such as read/write circuit 852 in FIG. 8C, thattakes as a control input the vector stored in the page register. The twobit vector stored in the page register may also correspond with a firstbit associated with whether the memory cell is deemed strong or typicaland a second bit associated with whether the memory cell is deemed to beat a near end of a word line.

In step 912, a first bit line current of the plurality of bit linecurrent options is selected. In one embodiment, the first bit linecurrent may be selected based on the determination that the memory cellcomprises a “strong” memory cell. In another environment, the first bitline current may be selected based on the determination that the memorycell comprises a “strong” memory cell and is located within apredetermined distance from a word line driver driving a word lineconnected to the memory cell (e.g., the memory cell comprises a near bitbased on the word line address).

In step 914, the memory cell is programmed by applying the first bitline voltage to the memory cell and the first bit line current to thememory cell. In step 916, a state of the memory cell is verified todetermine whether the memory cell has reached a particular programmingstate.

FIG. 9B is a flowchart describing one embodiment of a process forgenerating a plurality of bit line voltages associated with differentcompensation options. The process described in FIG. 9B is one example ofa process for implementing step 902 in FIG. 9A. In one embodiment, theprocess of FIG. 9B may be performed by a memory chip, such as memorychip 102 in FIG. 1.

In step 922, a word line address, a bit line address, and a bit linevoltage associated with a memory operation are acquired. The bit linevoltage may correspond with an uncompensated bit line voltage to beapplied during the memory operation (e.g., 2V during a read operation).The memory operation may comprise a programming operation or a readingoperation. In step 924, a memory array zone is determined based on theword line address. The word line address may determine the distance of aselected bit line to a corresponding bit line driver or read/writecircuit biasing the selected line. In step 926, a bit line layer isdetermined based on the bit line address. In one embodiment, a memoryarray may comprise a monolithic three-dimensional memory array and thebit line layer may correspond with a memory layer of the monolithicthree-dimensional memory array.

In step 928, a memory cell orientation is determined based on the bitline address and the word line address. In some memory arrayarchitectures, the memory cell orientation may be determined basedsolely on the bit line address. The memory cell orientation maycorrespond with a diode polarity or diode orientation (e.g., an upwardpointing diode). In step 930, a compensated bit line voltage isgenerated based on the bit line voltage, the memory array zone, the bitline layer, and the memory cell orientation.

In step 932, one or more memory cell dependent voltage offsets areacquired. The one or more memory cell dependent voltage offsets mayinclude an offset corresponding with whether a memory cell comprises astrong memory cell, a weak memory cell, or a typical memory cell. In oneexample, the bit line voltage offset for a memory cell that is deemed tobe a strong memory cell may correspond with a reduction in the bit linevoltage by 200 mV during a read operation. In another example, the bitline voltage offset for a memory cell that is deemed to be a strongmemory cell may correspond with an increase in the bit line voltage by500 mV during a programming operation.

In step 934, a plurality of bit line voltage options is generated basedon the compensated bit line voltage and the one or more memory celldependent voltage offsets. In one embodiment, the plurality of bit linevoltage options may be generated using a plurality of voltage regulatorseach generating an output voltage corresponding with the compensated bitline voltage and one of the one or more memory cell dependent voltageoffsets. In step 936, the plurality of bit line voltage options isoutputted. The plurality of bit line voltage options may be distributedacross a plurality of read/write circuits used for generating bit linevoltages during the memory operation.

FIG. 9C is a flowchart describing one embodiment of a process fordetermining whether a memory cell has characteristics of a strong memorycell, a weak memory cell, or a typical memory cell. The processdescribed in FIG. 9C is one example of a process for implementing step906 in FIG. 9A. In one embodiment, the process of FIG. 9C may beperformed by a memory chip, such as memory chip 102 in FIG. 1.

In step 942, a state of the memory cell is determined. The state of thememory cell may be determined via one or more memory cell sensingoperations. The state of the memory cell may correspond with aprogrammed state of the memory cell (e.g., an ON state of the memorycell). In step 944, it is determined whether an output currentassociated with the memory cell is below (or less than) a lowerthreshold based on the state. In one example, the state of the memorycell may correspond with the ON state depicted in FIG. 7B and the lowerthreshold may correspond with the current level IB depicted in FIG. 7B.In step 946, a weak bit corresponding with whether the output currentwas determined to be below the lower threshold is stored. The weak bitmay be stored in a page register.

In step 948, it is determined whether the output current associated withthe memory cell is above (or greater than) an upper threshold based onthe state. In one example, the state of the memory cell may correspondwith the ON state depicted in FIG. 7B and the upper threshold maycorrespond with the current level IA depicted in FIG. 7B. In step 950, astrong bit corresponding with whether the output current was determinedto be above the upper threshold is stored. The strong bit may be storedin a page register. In some cases, if neither the strong bit nor theweak bit are set to “1,” then the memory cell may be deemed a typicalmemory cell.

FIG. 9D is a flowchart describing one embodiment of a process forreading a memory cell. In one embodiment, the process of FIG. 9D isperformed by a memory chip, such as memory chip 102 in FIG. 1.

In step 962, a plurality of bit line voltage options associated with aread operation is generated. The plurality of bit line voltage optionsmay be generated using a bit line voltage options generator, such as VBLoptions generator 801 in FIG. 8A. In one embodiment, the plurality ofbit line voltage options may comprise two different bit line voltagescorresponding whether a memory cell is located at a far end of a wordline or not located at the far end of the word line. One embodiment of aprocess for generating a plurality of bit line voltages associated withdifferent compensation options was described previously in reference toFIG. 9B.

In step 964, a plurality of bit line current options associated with theread operation is generated. The plurality of bit line current optionsmay be generated using control circuitry similar to the VBL settingsgenerator 802 of FIG. 8A, but instead of binary values for differentvoltage settings being generated, the binary values may be generated fordifferent current settings. A plurality of reference currentscorresponding with the different current settings may be generated usinga plurality of configurable current mirrors. Each of the plurality ofconfigurable current mirrors may multiply a stable reference current(e.g., provided by a bandgap-based current reference) by some valuedepending on a given input current setting. In one embodiment, theplurality of bit line current options may comprise two different bitline currents corresponding with a strong memory cell and a weak memorycell.

In step 966, it is determined whether the memory cell is associated witha far end of a word line. In one embodiment, the determination ofwhether the memory cell is located at a far end of the word line may bedetermined based on a word line address associated with the memory cell.In other embodiments, it may be determined whether the memory cell isassociated with a far-far memory cell (i.e., a memory cell withrelatively large path resistance due to being located far from the bitline biasing circuit and located far from the word line biasing circuit)based on a word line address and a bit line address associated with thememory cell. In some cases, a memory cell may be deemed to be far fromthe end of a word line if it is located more than a certain distancefrom a word line driver driving the word line.

In step 968, a first bit line voltage of the plurality of bit linevoltage options is selected. In one embodiment, the first bit linevoltage may be selected based on the determination that the memory cellcomprises a “strong” memory cell. In another environment, the first bitline voltage may be selected based on the determination that the memorycell comprises a “strong” memory cell and is located more than apredetermined distance from a word line driver driving a word lineconnected to the memory cell (e.g., the memory cell comprises a far bitbased on the word line address).

In some cases, a page register may store a one bit or two bit vectorassociated with characteristics of the memory cell, such as whether thememory cell has been determined to be a strong, weak, or typical memorycell. The selection of the first bit line voltage may be performed usinga read/write circuit, such as read/write circuit 852 in FIG. 8C, thattakes as a control input the vector stored in the page register. The twobit vector stored in the page register may also correspond with a firstbit associated with whether the memory cell is deemed strong or typicaland a second bit associated with whether the memory cell is deemed to beat a near end of a word line.

In step 970, a first bit line current of the plurality of bit linecurrent options is selected. In one embodiment, the first bit linecurrent may be selected based on the determination that the memory cellcomprises a “strong” memory cell. In another environment, the first bitline current may be selected based on the determination that the memorycell comprises a “strong” memory cell and is located more than apredetermined distance from a word line driver driving a word lineconnected to the memory cell (e.g., the memory cell comprises a far bitbased on the word line address).

In step 972, a state of the memory cell is determined by applying thefirst bit line voltage to the memory cell and the first bit line currentto the memory cell. The state of the memory cell may be determined byperforming a sensing or read operation on the memory cell using aread/write circuit, such as read/write circuit 852 in FIG. 8C. In step974, data associated with the state of the memory cell is outputted. Thedata associated with the state of the memory cell may be outputted to apage register for buffering prior to transmission of the data outside ofa memory chip.

One embodiment of the disclosed technology includes generating aplurality of bit line voltage options associated with a programmingoperation, determining a state associated with a memory cell,determining an upper current threshold based on the state, determining afirst control bit based on whether an output current associated with thememory cell is greater than the upper current threshold, selecting afirst bit line voltage of the plurality of bit line voltage optionsbased on the first control bit, and programming the memory cell byapplying the first bit line voltage to the memory cell.

One embodiment of the disclosed technology includes a plurality ofvoltage generators, a plurality of memory cells including a first memorycell, and a first memory cell programming circuit in communication withthe first memory cell. The plurality of voltage generators generates aplurality of bit line voltage options associated with a programmingoperation. The first memory cell programming circuit determines a firststate associated with the first memory cell and determines an uppercurrent threshold based on the first state. The first memory cellprogramming circuit determines a first control bit based on whether anoutput current associated with the first memory cell is greater than theupper current threshold. The first memory cell programming circuitselects a first bit line voltage of the plurality of bit line voltageoptions based on the first control bit and programs the first memorycell by applying the first bit line voltage to the first memory cellduring the programming operation.

One embodiment of the disclosed technology includes generating aplurality of bit line voltage options associated with a read operation,generating a plurality of bit line current options associated with theread operation, and determining a first control bit. The determining afirst control bit includes acquiring a bit line address associated witha memory cell and determining whether the memory cell is located at afar end of a word line connected to the memory cell based on the bitline address. The method further includes selecting a first bit linevoltage of the plurality of bit line voltage options based on the firstcontrol bit, selecting a first bit line current of the plurality of bitline current options based on the first control bit, determining a stateof the memory cell by applying the first bit line voltage and the firstbit line current to a bit line connected to the memory cell, andoutputting data associated with the state of the memory cell.

For purposes of this document, each process associated with thedisclosed technology may be performed continuously and by one or morecomputing devices. Each step in a process may be performed by the sameor different computing devices as those used in other steps, and eachstep need not necessarily be performed by a single computing device.

For purposes of this document, reference in the specification to “anembodiment,” “one embodiment,” “some embodiments,” or “anotherembodiment” may be used to described different embodiments and do notnecessarily refer to the same embodiment.

For purposes of this document, a connection can be a direct connectionor an indirect connection (e.g., via another part).

For purposes of this document, the term “set” of objects, refers to a“set” of one or more of the objects.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims.

What is claimed is:
 1. A monolithic three-dimensional integratedcircuit, comprising: a three-dimensional memory array including a firstlayer of memory cells and a second layer of memory cells, the firstlayer of memory cells includes a first memory cell, the second layer ofmemory cells includes a second memory cell, the first memory cell islocated above the second memory cell, the second memory cell is locatedabove a substrate, the first memory cell and the second memory cell areformed above the substrate without any intervening substrates betweenthe first memory cell and the second memory cell; and supportingcircuitry in communication with the three-dimensional memory array, thesupporting circuitry generates a plurality of bit line voltage optionsassociated with a programming operation, the supporting circuitrydetermines a state associated with the first memory cell and determinesan upper current threshold based on the state, the supporting circuitryselects a first bit line voltage of the plurality of bit line voltageoptions based on whether an output current associated with the firstmemory cell is greater than the upper current threshold, the supportingcircuitry causes the programming operation to be performed to the firstmemory cell, the first bit line voltage is applied to the first memorycell during the programming operation.
 2. The monolithicthree-dimensional integrated circuit of claim 1, wherein: the firstmemory cell and the second memory cell are arranged in a vertical columnthat is perpendicular to the substrate.
 3. The monolithicthree-dimensional integrated circuit of claim 1, wherein: the firstmemory cell and the second memory cell are located within a verticalplane that is perpendicular to the substrate.
 4. The monolithicthree-dimensional integrated circuit of claim 1, wherein: the firstmemory cell and the second memory cell are in communication with a firstbit line, the first bit line is arranged in a vertical direction that isperpendicular to the substrate, the first bit line voltage is applied tothe first bit line during the programming operation.
 5. The monolithicthree-dimensional integrated circuit of claim 1, wherein: the firstlayer of memory cells includes a third memory cell, the second layer ofmemory cells includes a fourth memory cell, the third memory cell islocated above the fourth memory cell, the first memory cell and thethird memory cell are arranged in a first horizontal plane, the secondmemory cell and the fourth memory cell are arranged in a secondhorizontal plane that is below the first horizontal plane.
 6. Themonolithic three-dimensional integrated circuit of claim 1, wherein: thethree-dimensional memory array and the supporting circuitry areintegrated within the monolithic three-dimensional integrated circuit.7. The monolithic three-dimensional integrated circuit of claim 6,wherein: the three-dimensional memory array is fabricated above thesupporting circuitry.
 8. The monolithic three-dimensional integratedcircuit of claim 1, wherein: the supporting circuitry determines whetherthe first memory cell is located at a near end of a word line connectedto the first memory cell, the supporting circuitry selects the first bitline voltage of the plurality of bit line voltage options based onwhether the first memory cell is located at the near end of the wordline.
 9. The monolithic three-dimensional integrated circuit of claim 1,wherein: the supporting circuitry selects the first bit line voltage ofthe plurality of bit line voltage options based on whether the firstmemory cell comprises a near-near memory cell.
 10. The monolithicthree-dimensional integrated circuit of claim 1, wherein: the supportingcircuitry generates the plurality of bit line voltage options based onat least one of a memory array zone associated with the first memorycell or a memory cell direction associated with the first memory cell.11. A method for operating a monolithic three-dimensional integratedcircuit, comprising: generating a plurality of bit line voltage optionsassociated with a programming operation for a three-dimensional memoryarray, the three-dimensional memory array includes a first memory celland a second memory cell arranged in a vertical column that isperpendicular to a substrate, the first memory cell is located above thesecond memory cell, the second memory cell is located above thesubstrate; determining a state associated with the first memory cell;determining an upper current threshold based on the state; selecting afirst bit line voltage of the plurality of bit line voltage optionsbased on whether an output current associated with the first memory cellis greater than the upper current threshold; and applying the first bitline voltage to the first memory cell during the programming operation.12. The method of claim 11, wherein: the first memory cell and thesecond memory cell are in communication with a first bit line, the firstbit line is arranged in a vertical direction that is perpendicular tothe substrate, the first bit line voltage is applied to the first bitline during the programming operation.
 13. The method of claim 11,wherein: the three-dimensional memory array includes a third memory celland a fourth memory cell, the third memory cell is located above thefourth memory cell, the first memory cell and the third memory cell arearranged in a first horizontal plane, the second memory cell and thefourth memory cell are arranged in a second horizontal plane that isbelow the first horizontal plane.
 14. The method of claim 11, wherein:the selecting a first bit line voltage includes selecting the first bitline voltage based on whether the first memory cell is located at a nearend of a word line connected to the first memory cell.
 15. The method ofclaim 11, wherein: the generating a plurality of bit line voltageoptions includes generating the plurality of bit line voltage optionsbased on at least one of a memory array zone associated with the firstmemory cell or a memory cell direction associated with the first memorycell.